Writing down the weather

01 Jun 2019 The Navigator

Huw Davies, Principal of Meteorology at Stratum Five Ltd., a British company supplying vessel monitoring and fleet tracking technology, examines how and why to accurately record weather conditions at sea

“Some are weather wise, some are otherwise” – Benjamin Franklin

Seafarers are generally very ‘weather wise’. All Masters are required by SOLAS to issue a danger message when extreme weather conditions are encountered, namely ice accretion, icebergs, storm force winds and tropical cyclones. More generally, the weather is recorded at each watch in the ship’s log. Some vessels, known as Voluntary Observing Ships (VOS), also report their observations to the World Meteorological Organization (WMO) network of forecasting centres.

It is good seamanship to be aware of the major pressure patterns, METAREA forecast and expected conditions and to constantly compare what is predicted with what you are experiencing. Rising or falling barometric pressure and wind speed and direction give an indication of the location and movement of the major weather systems. The sequence of clouds can indicate the approach of a frontal system. Increasing and lowering stratiform (layered) clouds herald the approach of a warm front.

Logging the weather
Despite the increasing prevalence of onboard sensors and data streaming to handling centres ashore for analysis, humans still play a critical role in observing the weather. Some phenomena, such as ice accretion and visibility, are not well captured by instruments. Sensors, particularly anemometers, can be inaccurate due to poorly sighted or un-calibrated instruments. Perhaps the most important reason for taking weather observations on board is that the ship’s log is still the preferred source of information in the case of any disputes or accidents.

What to include?
A good log entry should include:
Pressure – If possible, record the barometric pressure and whether pressure is increasing, steady or decreasing. Note that if the sensor is in a fairly airtight space, e.g. the bridge, this may affect the reading.
Wind – The true wind speed and direction should be recorded, as opposed to the relative wind passing over the deck. Wind can be estimated by studying the sea state. It is good practice to keep a Beaufort scale with associated sea conditions on the bridge. If using an anemometer, readings should be averaged over ten minutes.Take care if doing this by watching the dial of the anemometer, as it is not uncommon to overestimate wind speed by more than ten percent.
Weather – Note relevant conditions, such as precipitation and lightning.
Visibility – Estimate and record visibility using the internationally agreed definitions:

Very poor or fog Visibility less than 1,000 m
Poor Visibility between 1,000 m and 2 nautical miles
Moderate Visibility between two and five nautical miles
Good Visibility more than five nautical miles

If other shipping is present, radar ranges can be used to accurately assess visibility.
Sea state and swell – Waves generated by a wind that is blowing are referred to as ‘sea’ or ‘sea state’. When the wind stops or changes direction, waves that continue on without relation to local winds are called ‘swell’.

Swell is of particular interest to seafarers because it can affect the ship’s intact stability and lead to broaching, parametric or synchronous rolling.
When recording sea state, it is advised to use the terminology in the following table:

Smooth Wave height less than 0.5 m
Slight Wave height of 0.5 to 1.25 m
Moderate Wave height of 1.25 to 2.5 m
Rough Wave height of 2.5 to 4.0 m
Very rough Wave height of 4.0 to 6.0 m
High Wave height of 6.0 to 9.0 m
Very high Wave height of 9.0 to 14.0 m
Phenomenal Wave height more than 14.0 m

For sea swell, record the length (‘short’ 0-100m, ‘average’ 100-200m, ‘long’ over 200m), height (‘low’ 0-2m, ‘moderate’ 2-4m, ‘heavy’ over 4m) and the true direction.
More guidance on the observation of waves and swell, as well as the observation of sea ice, can be found in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8) Part II, Chapter 4, Marine Observations.

Anything else that is noteworthy for example, ship-handling characteristics.

Taking away the guess work

Petty Leung FNI, Managing Director APAC of weather forecasting service provider, StormGeo Ltd., reveals how modern shorebased weather services are helping mariners predict the weather more accurately than ever before

Shore-based weather services have been available to mariners since the early 1950s. They have come a long way since their initial form of route recommendation which was often met with scepticism and dismissal as no more than a ‘rent-a-storm’ service. Today, services are far more sophisticated with a wide range of critical weather-related data available to help mariners prepare for all eventualities during their voyage.

On the meteorological and environmental data front, forecast performance has certainly improved. Modern forecasting technology tracks tropical cyclones and typhoons, giving mariners a much clearer window on the possible hazards that could arise during a forthcoming journey. Another significant advance is the use of ensemble forecasting. Instead of giving a single forecast at a given time, a ‘set’ of forecasts is produced, aiming to give an indication of the possible future status of the atmosphere. In addition, the availability of dynamic ocean current forecasts that offer up to three days of predicted data are a significant aid to fuel efficiency by allowing the fine-tuning of routes to take advantage of predicted ocean currents. ‘Catching the current’ has become more of a science than simple guess work.

Technology today
Today’s availability and improvement in accuracy of weather data is enhanced by technological advances. Satellite communication has become more efficient and affordable. Computers are widely available to vessels. On-board weather routing systems with sophisticated speed-down algorithms have been developed. Interactive tools simulate the best route, based on up to 16 days weather forecasting, along with relevant navigation constraints and commercial requirements for the voyage.

Mariners can have a day-by-day view of the weather conditions for their passage ahead, which enables them to calculate realistic estimates of passage time and fuel consumption. Not only does this help place vessels strategically in the best part of the ocean, mariners can also plan tactically how to achieve safe navigation with optimum fuel efficiency.
Most recently, electronic chart planning is being combined with on-board weather routing systems on a single platform. This enables mariners to optimise passage plans to take account of weather routing with the ease of matching electronic charts. This integration ensures the voyage is safe, navigationally sound and fuel efficient.

Taking back control
The importance of the top-quality weather services now available to mariners is highlighted by the huge impact that improved weather forecasting performance, technology and the quest for efficiency can have on a voyage. Whether there is a raging storm on the horizon or a fleeting encounter with a monsoon, timely information and smart tools that assess the effect of predicted weather can help navigators plan a calm voyage, plot an alternative and prepare the ship and her crew to brave any heavy weather ahead.

The wide range of modern weather services that are now available not only enable mariners to be informed and pre-emptive, but they give back that all-important element of control.